1. Field of the Invention
The present invention relates generally to resistors for dividing a high voltage in cathode ray tubes, and more particularly, to a resistor used in combination with an electron gun assembly in a cathode ray tube for breeding relatively high voltages required to be applied to the electrodes of the electron gun assembly.
2. Description of the Prior Art
There has been proposed a color cathode ray tube used in a color television receiver in which relatively high voltages are required to be supplied to convergence electrodes for converging a plurality of electron beams, focus electrodes for focusing each of the electron beams and so on, in addition to an anode voltage. In such a color cathode ray tube, a resistor is used in combination with an electron gun assembly containing the convergence electrodes, focus electrodes and other electrodes for dividing the anode voltage to breed the relatively high voltages supplied to the respective electrodes.
A previously proposed resistor for use in the color cathode ray tube in the manner as mentioned above is shown in FIGS. 1 and 2. FIG. 1 is a plane view showing a resistor 7 previously proposed with a major part thereof shown through a coating insulator forming an exterior portion, and FIG. 2 is a side view showing the resistor 7 entirely. The resistor 7 has an insulating plate 1 made of, for example, ceramics and provided with a plurality of terminals formed by separate conductive layers put on the surface thereof, respectively. These terminals contain an anode electrode terminal 2 for being supplied with the anode voltage, a convergence electrode terminal 3 for delivering the relatively high voltage supplied to the convergence electrodes, that is, a convergence voltage, and an earth electrode terminal 4. Then, a voltage dividing resistive layer 5 is also put on the surface of the insulating plate 1. This voltage dividing resistive layer 5 comprises a partial resistive layer 5a formed in the zigzag pattern with a predetermined resistance to connect the convergence electrode terminal 3 with the earth electrode terminal 4, another partial resistive layer 5b formed in the zigzag pattern also with a predetermined resistance to connect the anode electrode terminal 2 with the convergence electrode terminal 3, and an adjusting resistive layer 5c provided to couple with the convergence electrode terminal 3 and the partial resistive layers 5a and 5b. The resistance of each of the partial resistive layers 5a and 5b can be adjusted by removing the adjusting resistive layer 5c partially in the manufacturing process of the resistor 7. Further, at the hatched portion on the insulating plate 1, a coating insulator 6 of, for example, flint glass is provided to cover the voltage dividing resistive layer 5.
The resistor 7 thus constituted is used in a color cathode ray tube in such a manner as illustrated in FIG. 3. In FIG. 3, an electron gun assembly 9 is disposed in a neck portion 8a of a body of tube 8 of the color cathode ray tube, and has three cathodes K, an arrangement of a first grid electrode G1, a second grid electrode G2, a third grid electrode G3, a fourth grid electrode G4 and a fifth grid electrode G5 aligned in common to the three cathodes K, and convergence electrodes 10 provided next to the fifth grid electrode G5. The first to fifth grid electrodes G1 to G5 and the convergence electrodes 10 are connected mechanically with a beading glass 11 to be supported in common thereby, and the third and fifth grid electrodes G3 and G5 are coupled electrically with each other through a conductive wire 13. The convergence electrodes 10 comprises a pair of inner deflecting electrode plates 10a and 10b faced to each other and connected electrically to the fifth grid electrode G5 through a conducting plate 14 and a pair of outer deflecting electrode plates 10c and 10d provided to face to the inner deflecting electrode plates 10a and 10b, respectively.
The resistor 7 as shown in FIGS. 1 and 2 is attached to the electron gun assembly 9 with the anode electrode terminal 2 connected through a conductive connecting piece 12 to the fifth grid electrode G5. On the inner surface of a funnel portion 8b of the body of tube 8, a graphite coating 15 is provided to extend to the inner surface of the neck portion 8a, and the anode voltage is applied through a high voltage supplying button, that is, an anode button (not shown in Figures) built in the funnel portion 8b to the graphite coating 15. The conducting plate 14 is provided with conductive springs 16 which come into contact with the graphite coating 15 so that the anode voltage is supplied to the fifth grid electrode G5, the third grid electrode G3, the inner deflecting electrode plates 10a and 10b of the convergence electrodes 10 and the anode electrode terminal 2 of the resistor 7. The convergence electrode terminal 3 of the resistor 7 is connected through a conductive connecting piece 17 to the outer deflecting electrode plates 10c and 10d of the convergence electrodes 10 and the earth electrode terminal 4 of the resistor 7 is connected with an earth electrode terminal pin 19 fixed through a stem portion 18 at the end of the neck portion 8a of the body of tube 8 to be grounded directly or through a variable resistor provided in the outside of the body of tube 8, so that the convergence voltage obtained at the convergence electrode terminal 3 as a result of the division of the anode voltage by the partial resistive layers 5a and 5b is supplied to the outer deflecting electrode plates 10c and 10d of the convergence electrodes 10.
In the color cathode ray tube containing the electron gun assembly 9 and the resistor 7 therein as mentioned above, if the electron gun assembly 9 has sharp-pointed projections thereon, undesirable electric discharge may occur at some of the sharp-pointed projections in actual use. Accordingly, the color cathode ray tube is subjected to the knocking treatment in the manufacturing process thereof in which such portions as the sharp-pointed projections on the electron gun assembly 9 where electric discharge is likely to occur are caused positively to have electric discharge thereat previously to be reformed with melting away, for the purpose of stabilizing the operation thereof in practical use. In the knocking treatment, the third and fifth grid electrodes G3 and G5 of the electron gun assembly 9 and the anode electrode terminal 2 of the resistor 7 are supplied with a high voltage (knocking voltage) which is twice to third times as high as the anode voltage in the practical use of the color cathode ray tube, and the first, second and fourth grid electrodes G1, G2 and G4 are grounded.
In the situation of such knocking treatment, the outer surface of the coating insulator 6 forming the exterior of the resistor 7 is electrically charged to be at relatively high potential except for a certain part thereof, and the coating insulator 6 is applied a voltage higher than that in the practical use of the cathode ray tube particularly on the low voltage side of the partial resistive layer 5a. FIG. 4 shows the potential on the outer surface of the coating insulator 6 and the potential on the partial resistive layer 5a provided between the earth electrode terminal 4 and the convergence electrode terminal 3 under the knocking treatment with curves a and b, respectively, and further the difference between the potentials shown with the curves a and b, respectively, with a curve c in the graphic illustration having the axis of ordinates representing voltage V and the axis of abscissas representing distance L measured on the surface of the insulating plate 1 from the earth electrode terminal 4 toward the convergence electrode terminal 3 of the resistor 7 and shown with reference to the resistor 7 and the electron gun assembly 9. As apparent from this illustration in FIG. 4, the potential difference between the partial resistive layer 5a and the outer surface of the coating insulator 6 reaches the maximum at a position P close to the third grid electrode G3 supplied with the knocking voltage on the low voltage side of the partial resistive layer 5a, and therefore, the maximum voltage is applied to the coating insulator 6 at the position P. Consequently, it is feared that a voltage exceeding the upper limit of the resistible voltage for the coating insulator 6 is applied to the coating insulator 6 at the position around the third grid electrode G3 of the electron gun assembly 9 so as to bring deterioration in dielectric strength or dielectric breakdown on the coating insulator 6 and, as a result, the partial resistive layer 5a is damaged to vary its resistance conspicuously.
Against such variations in the resistance of partial resistive layer 5a resulting from the deterioration in dielectric strength or dielectric breakdown brought on the coating insulator 6 as mentioned above, it may be advantageous that the coating insulator 6 is given an increased thickness to have raised dielectric strength. That is, it is possible to prevent the deterioration in dielectric strength or dielectric breakdown from being brought on the coating insulator 6 and thereby to restrain the variations in the resistance of partial resistive layer 5a by means of making the coating insulator 6 have an increased thickness.
However, it is disadvantageous for the production cost of the resistor 7 to increase the thickness of the coating insulator 6 indiscreetly. Further, the coating insulator 6 with the increased thickness may cause the problem that the resistor 7 is undesirably warped due to difference in the coefficient of thermal expansion between the insulating plate 1 and the coating insulator 6, and the coating insulator 6 comes to exfoliate from the insulating plate 1 or comes to be cracked through the repetition of an increase in temperature of the resistor 7 in the operative state and a decrease in temperature of the resistor 7 in the inoperative state occuring alternately. This results in that the reliability of the resistor 7 is lowered.